Various types of exhaust gas purifying systems, for purifying the exhaust gas emitted from an internal combustion engine, are used according to the properties of the exhaust gas, and many of them trap a poisonous component contained in the exhaust gas and require regeneration operation at appropriate time intervals.
For example, an exhaust gas purifying system, for a gasoline engine or, especially, for a gasoline engine which executes lean burning for most of the running period, except accelerating periods or the like, which absorbs nitrogen oxide for the lean burning period and releases the absorbed nitrogen oxide for the short rich burning period has already proposed (Refer to International Publication No. W093/07363).
The above-mentioned exhaust purifying system absorbs nitrogen oxide contained in the exhaust gas in an absorbent incorporated in the exhaust gas purifying system for the lean burning period, which accounts for most of the running period, and executes a regeneration operation which releases the nitrogen oxide from the absorbent by increasing an amount of fuel to make air-fuel ratio rich when it is determined that the absorbing power has deteriorated. Note, nitrogen oxide released during the regeneration operation is not directly emitted into the air because it is deoxidized by unburned hydrocarbon and carbon monoxide and converted to nitrogen gas, carbon dioxide and water in the exhaust gas with rich air-fuel ratio. The nitrogen oxide released for the regeneration operation is dioxidized by unburned hydrocarbon in the exhaust gas, the air-fuel ratio thereof being rich, and thus is converted into nitrogen gas, carbon dioxide and water. The nitrogen oxide, therefore, is not emitted into the air.
However, if the remaining absorbing power of the absorbent is evaluated based on the integrated value of the product of the intake air flow rate which is proportional to the amount of nitrogen dioxide in the exhaust gas and the engine load or in a more simplistic manner, based on the integrated value of the engine speed, and a regeneration operation is executed when it is determined that the remaining absorbing power decreases below a fixed level, it is not avoidable that the nitrogen oxide is emitted in the air depending on the exhaust gas temperature.
Because the air-fuel ratio in the absorbent gradually changes from the lean state to the rich state when the air-fuel ratio is controlled from the lean state to the rich state, nitrogen oxide is not completely deoxidized and is liable to be emitted in the air before the actual air-fuel ratio has been completely transferred to the rich state.
However, it is known that the discharge amount of nitrogen oxide when switching the air-fuel ratio from rich to lean depends mainly on the temperature of the absorbent and becomes almost zero when the temperature of the absorbent is lower than 200.degree. C.
Therefore, the present applicant has already proposed stopping the regeneration operation as long as the absorbent has a residual absorption power when the temperature of the absorbent (or the exhaust gas) rises over the temperature where the regeneration operation is permitted.
However, the regeneration operation of the exhaust gas purifying system based on the current temperature of the absorbent is not always optimal.
Namely, even if the regeneration timing of the exhaust gas purifying system is controlled based on the current temperature of the exhaust gas in addition to the integrated value of the engine speed, the nitrogen oxide is liable to be emitted into the air when the exhaust gas temperature suddenly rises while the exhaust gas purifying system is regeneration, and when the exhaust gas temperature becomes the temperature where regeneration operation is permitted while the nitrogen oxide is being absorbed, the fuel consumption may unnecessarily increase to enrich the air-fuel ratio.
On the other hand, in an exhaust gas purifying system for a diesel engine, the particulate filter thereof, for trapping particulates (carbon particles) contained in the exhaust gas, is periodically regenerated as has been proposed (Japanese Unexamined Patent Publication (Kokai) No. 1-318715).
Namely, for the diesel engine, a particulate filter is installed in the exhaust system in order to remove the particulates from the exhaust gas before the gas is emitted into the atmosphere. Because the trapping power of the particulate filter is limited, it is necessary to remove the particulate at appropriate times.
Therefore, the above described exhaust gas purifying system proposes not only to promote the spontaneous burning off of the particulates by adding catalyst in the particulate filter, but also the burning-off of the particulates by continuing the reaction between the particulates and the nitrogen dioxide which is converted from nitrogen monoxide, by an oxidation catalyst installed upstream of the particulate filter, when the exhaust gas temperature is not so high.
Because conversion of nitrogen monoxide into nitrogen dioxide by the oxidation catalyst, however, requires that the exhaust gas temperature is within a predetermined range, the particulates cannot be burned off when the exhaust gas temperature is out of the predetermined range.
Therefore, it has been proposed to promote the burning off of the particulates by raising the exhaust gas temperature into the predetermined temperature range, by heating with a light oil burner or an electric heater, by closing the intake air, or by combining the above, when it is determined that the exhaust gas temperature is out of the predetermined range based on the rotational speed and the load of the diesel engine.
Nevertheless, when the exhaust gas temperature shifts into the predetermined range and it becomes possible to remove the particulate without heating by the light oil burner or the like after it is determined that the heating with the light oil burner or the like is necessary because the exhaust gas temperature is out of the predetermined range, the fuel consumption rate is unavoidably deteriorated due to the unnecessary heating.